mixed solvent dielectric

[MARYJO.,at,.neu.edu]

 Hello!  Please help me.  I need information
 about the determination of the dielectric
 constant of a solvent mixture (e.g. DMF,
 DMSO, acetnitrile and water, maybe 0-40%
 solvent in water).  Some of these solvents
 canhave ion-pairing interactions.  I
 believe that the weighted average dielectic
 of the two solvents is not a good enough
 approximation.
 Can anyone advise me - point me to the
 right sources.
 Thanks very much,
 Mary Jo Ondrechen